Question

Predict why the refractive index of air is slightly higher than the refractive index of a vacuum?

Answer 1

The refractive index of a medium is defined as `n=c/v`, where `c` is the speed of light in a vacuum and `v` is the apparent velocity of light traveling through that medium.

The refractive index of a vacuum is `c/c=1`.

Since light seems to travel slower in air (but not by much since air is not as optically dense), the refractive index will be greater `c/v>1` since `0< v< c`.

Note that light does not actually slow down when traveling through a medium. Its actual speed is still `c`. Read more about this here on Wikipedia.